Class-D audio amplifiers are desirable in audio systems because they are efficient and because they can handle high power signals. The high efficiency allows for smaller power supplies and smaller heat sinks. Typically, a switching output stage of a Class-D amplifier is driven by a pulse width modulation (PWM) modulator that outputs a one-bit signal that represents two voltage levels that can be output by the switching output stage. The one-bit voltage signal output by the switching output stage is typically filtered through a simple low-pass filter (LPF) and driven into a speaker.
FIG. 1 is a schematic block diagram illustrating an open-loop digitally-driven Class-D amplifier 100 according to the prior art. In FIG. 1, digital PWM modulator 104 converts the signal output by delta-sigma modulator (DSM) 102 to a PWM signal. The PWM signal output by digital PWM modulator 104 is used by drivers 106 to control the output stage of a Class-D amplifier. The open-loop digitally-driven Class-D amplifier illustrated in FIG. 1 has numerous drawbacks. For example, the non-idealities of the power switches of drivers 106, such as dead time, signal-dependent delays, and slew rate, act as sources of distortion and may add extra noise to the overall Class-D system. In addition, open-loop digitally-driven Class-D amplifiers have a near-zero Power Supply Rejection Ratio (PSRR) for a non-regulated power supply. Low PSRR leads to possible audible artifacts.
Analog closed-loop Class-D amplifiers have been used to address some issues present in open-loop digitally-driven Class-D amplifiers such as shown in FIG. 1. FIG. 2A is a schematic block diagram illustrating a closed-loop analog-driven Class-D amplifier 200 according to the prior art. In FIG. 2A, an analog PWM modulator (Delta-Sigma Modulator 202) receives an analog input voltage VIN and receives a feedback voltage fed back from a PWM output power stage 204. The difference between the analog input voltage VIN and the feedback voltage is passed through a loop filter of the Delta-Sigma Modulator 202, which includes integrators and a PWM conversion block, followed by the PWM output power stage 204. The closed-loop nature of this topology allows for reduction in distortion from the non-idealities of the power switches, such as dead time, signal-dependent delays, and slew rate, get corrected. In addition, the PSRR of the overall Class-D system illustrated in FIG. 2A is improved over the open-loop Class-D system illustrated in FIG. 1. Therefore, a non-regulated power supply ripple is no longer an issue.
One drawback associated with the closed-loop analog-driven Class-D amplifier 200 is that a digital-to-analog converter (DAC) is needed to provide the analog input to the closed-loop analog-driven Class-D amplifier 200. This DAC is needed when, for example, a digital signal processor (DSP) is used to process data and generate the audio signal for input to the amplifier. After the audio signal has been processed, a DAC is used to convert the digital audio signal to a continuous-time voltage waveform that can be received by the closed-loop analog-driven Class-D amplifier 200.
FIG. 2B is a schematic block diagram illustrating another closed-loop analog-driven Class-D amplifier 250 according to the prior art in which the analog input signal of the analog PWM modulator of the Class-D amplifier is provided by a DAC. The DAC illustrated in FIG. 2B may be a switched-capacitor DAC (SC-DAC) or a current-steering DAC or a R-2R DAC. The DAC adds extra noise to the overall Class-D amplifier system and limits the optimization of the integrator gain of the analog PWM modulator. In addition, due to the switching nature of the feedback signal and the slow moving DAC output signal, the error signal between the input signal and the feedback signal are integrated through the integrators of the analog PWM modulator, which is illustrated in FIGS. 3A and 3B.
FIGS. 3A and 3B are plots illustrating signals processed within an analog PWM modulator of a Class-D amplifier according to the prior art. In FIG. 3A, the differential analog input signal provided as an input to the analog PWM modulator is represented by signals 312A and 312B and is shown as having a 1 V differential DC voltage. The differential feedback voltage signal that is fed back from a PWM output power stage, and that is also provided as an input to the analog PWM modulator, is represented in FIG. 3A by signals 314A and 314B. In FIG. 3B, signals 322A and 322B represent the output voltage signals of a first integrator of an analog PWM modulator that receives the input voltage signal and the feedback voltage signal, respectively. The output swing of the first integrator of the analog PWM modulator is represented by signal 324. As illustrated in FIG. 3B, the peak-to-peak output swing of signal 324 may be as high as 1 V. Because the gain of the first integrator of the analog PWM modulator is limited by an allowable inherent voltage swing at the output of the first integrator in the analog PWM modulator, the 1 V output swing that results from only the error signal between the input voltage signal and the feedback voltage signal limits the overall gain that can be used for the integrator.
FIGS. 3A and 3B illustrate that because the error between the input signal and the feedback signal is integrated through the integrators of the analog PWM modulator, the gain of the integrators is limited to prevent signal clipping. Because increasing the gains of the integrators allows reducing the resistance or its equivalent provides one mechanism for reducing the noise floor in a closed-loop analog-driven Class-D amplifier, the limitation on the gain as a result of the use of a DAC also limits the noise floor that can be achieved with the closed-loop analog-driven Class-D amplifier. In other applications, the size of the passive components of an electronic device may be increased to reduce the noise floor. However, in conventional Class-D amplifiers, such as the Class-D amplifiers illustrated in FIGS. 1 and 2A-B, scaling of the passive components sizes of Class-D amplifiers to reduce the noise floor is not feasible, because such scaling will increase power and silicon area beyond allowable limits.
Shortcomings mentioned here are only representative and are included simply to highlight that a need exists for improved electrical components, particularly for improved audio amplifiers, such as audio amplifiers that may be used in mobile devices. Embodiments described herein address certain shortcomings but not necessarily each and every one described here or known in the art.